When a Rock Key Gives a Confident Wrong Answer
A lab group has 12 rock samples and a dichotomous key that begins '1a. Dark-colored rock ... go to 2; 1b. Light-colored rock ... go to 5.' Sample 7 is fine-grained, dark gray, and magnetic; the key sends it to basalt. Sample 9 is coarse-grained, pink, and contains quartz, feldspar, and biotite; the key sends it to granite. Students also receive Sample 11: black, glassy, conchoidal fracture, no visible crystals, hardness about 5.5, and no reaction to 5% vinegar. The key has no glassy option, so students label it 'basalt.' The teacher knows Sample 7 is a fine-grained volcanic rock with abundant magnetite, Sample 9 is granite, and Sample 11 is obsidian. The class has 24 students working in pairs, 10-minute rotations, and only one streak plate per station.
1. Explain why the key misclassifies Sample 11 and why color alone is unreliable for these samples. What properties should the students use instead?
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The key misclassifies Sample 11 because its first split uses color, not texture or fracture. Obsidian is black and glassy with conchoidal fracture and no visible crystals; basalt is also dark but fine-grained and crystalline, may contain vesicles, and typically has a different luster. Color is a poor discriminator because the same mineral assemblage can vary with grain size and trace impurities; Sample 7's magnetism suggests magnetite, which does not make it basalt. Students should test luster, crystal size and visibility, fracture versus cleavage, hardness, reaction to acid, streak, and magnetism. A single property is rarely enough. The teacher can ask them to find a property that separates glassy from crystalline texture before using color.
2. The teacher wants students to revise the key rather than simply be told the right answers. Describe a revision that would distinguish Sample 11 from basalt and reduce errors for Samples 7 and 9.
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Revise the key to begin with texture and fracture: 'no visible crystals and conchoidal fracture' leads to obsidian; 'visible or microscopic crystals' leads to the crystalline rocks. Then split crystalline rocks by grain size and mineral composition, using color only after texture. Add a magnet test branch for Sample 7 and a note that a dark fine-grained rock can be basaltic andesite or another volcanic rock. For Sample 9, use 'coarse-grained with quartz, feldspar, and biotite' rather than pink color. Have pairs test the revised key on all 12 samples and compare disagreements; this makes the key's limits explicit.
3. What practical constraints in the lab could affect the reliability of the identification, and how should the teacher address them?
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Ten-minute rotations and one streak plate per station create queues and encourage rushed single tests. Small samples may be too small for a hardness scratch test without breaking, and scratching can damage specimens. The teacher should set up separate hardness, streak, acid, and magnet stations, provide duplicate streak plates, use larger reference samples, and give 15 to 20 minutes. Students should record each test before moving, and the teacher should spot-check two samples per group. Safety requires acid in drop bottles, goggles, and no taste test. These changes preserve the identification task while reducing avoidable error.